Magnetic recording disk files are information storage devices which utilize at least one rotatable disk with circular data tracks, a read/write head for reading and writing data to the various tracks, and a head positioning actuator connected to the head for moving it to the desired track and maintaining it over the track centerline during read or write operations. The read/write head is attached to an air bearing slider which is supported adjacent the data surface of the disk by a cushion of air generated by the rotating disk. The slider is attached to a generally flexible suspension which in turn is attached to the rigid support arm of the head positioning actuator.
In order to accurately position the read/write head radially on the disk, it is necessary to incorporate a servo control system which utilizes servo information previously magnetically recorded either on a dedicated servo disk or on sectors angularly spaced and interspersed among the data on a data disk. The servo information is sensed by the read/write head, or the dedicated servo head if a dedicated servo disk is used, and is demodulated to generate a position error signal which is an indication of the position error of the head away from the nearest track centerline. The use of magnetically recorded servo information reduces the amount of disk surface available for data, especially when an entire disk surface is required for the recorded servo information.
The loss of disk surface available for data can be prevented by a servo technique which involves sensing variations in capacitance of a predetermined capacitive contrast pattern formed on the same disk which contains the magnetically recorded data. The capacitive contrast servo pattern provides head position information for use by the servo control system. A magnetic recording disk containing a capacitive contrast pattern is described in assignee's published European patent application EP 125,478, which corresponds to assignee's co-pending application, U.S. Ser. No. 494,743, now abandoned. The structure of that disk is a conventional aluminum-magnesium substrate, a dielectric layer of epoxy phenolic resin formed on the substrate, a patterned aluminum film formed on the dielectric layer, and a conventional magnetic coating of an organic binder containing magnetic particulates formed over the patterned aluminum film. The presence or absence of aluminum in the patterned aluminum film serves as the capacitive contrast servo pattern. A different type of magnetic recording disk with a capacitive servo pattern is described in assignee's U.S. Pat. No. 4,556,597, wherein the capacitive contrast pattern comprises doped and undoped portions of the silicon disk substrate which supports the overlying magnetic coating. A capacitive sensor for use with suck disks is described in the IBM Technical Disclosure Bulletin, Vol. 26, No. 6 (Nov. 1983), pp. 2942-2946. This sensor utilized DC excitation of the capacitance to be measured. A capacitive sensor which utilizes an inductive element formed via thin film technology on the back side of the slider is described in assignee's U.S. Pat. No. 4,648,087. The variations in capacitance sensed by the sensor described in the '087 patent result in a change in the resonant frequency of the sensor circuit.